1. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to power control in a code division multiple access radiotelephone system.
2. Description of the Related Art
Multiple access modulation techniques are some of the most efficient techniques for utilizing the limited radio frequency spectrum. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum system uses a modulation technique that spreads the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal.
A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only part of a signal is typically affected by a frequency selective fade, the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, is affected less by the fade condition than a receiver using other types of signals.
The spread spectrum technique is accomplished by modulating each base band data signal to be transmitted with a unique wide band spreading code. Using this technique, a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. Typical examples of spread spectrum techniques are found in M. K. Simon, Spread Spectrum Communications, Volume I, pp. 262-358.
In a CDMA-type radiotelephone system, multiple signals are transmitted simultaneously at the same frequency. A particular receiver then determines which signal is intended for that receiver by a unique spreading code in each signal. The signals at that frequency, without the particular spreading code intended for that particular receiver, appear to be noise to that receiver and are ignored.
Since multiple radiotelephones and base stations transmit on the same frequency, power control is an important component of the CDMA modulation technique. A higher power output by a radiotelephone and/or base station increases the interference experienced by the other radiotelephones and base stations in the system. In order to keep the radiotelephones and base stations from transmitting at too much power, thereby lowering system capacity, some form of power control must be implemented.
The radiotelephone can aid the base station in the control of the power on the forward link (from the base station to the radiotelephone) by transmitting a power control message to the base station on the reverse link (from the radiotelephone to the base station). The radiotelephone gathers statistics of its error performance and informs the base station via a power control message. The base station may then adjust its power level to the specific user accordingly.
In a typical CDMA cellular communication system that follows the Telecommunications Industries Association/Electronic Industries Association Interim Standard 95 (IS-95), the base station adjusts its forward link power at a rate no faster than once per frame. This results in a high .sup.E b/N.sub.o on the forward link during low speed travel of the radiotelephone, due to the effects of Rayleigh fading, since the radiotelephone remains in the fade longer.
The ratio .sup.E b/N.sub.o is a standard quality measurement for digital communications system performance. The ratio expresses the bit-energy-to-noise-density of the received signal. .sup.E b/N.sub.o can be considered a metric that characterizes the performance of one communication system over another; the smaller the required .sup.E b/N.sub.o the more efficient is the system modulation and detection process for a given probability of error. A more detailed discussion of this concept can be seen in B. Sklar, Digital Communications, Fundamentals and Applications, Chapter 3 (1988).
A related metric is the .sup.E s/N.sub.o, which is the ratio of symbol-energy-to-noise-density of the received signal. The .sup.E s/N.sub.o, is related to the .sup.E b/N.sub.o by: ##EQU1##
where N is the number of bits per symbol. In binary phase shift keying (BPSK) modulated communication systems, N=1 and, consequently, .sup.E s/N.sub.o =.sup.E b/N.sub.o. In quadrature phase shift keying (QPSK) modulated communication systems, N=2 so that .sup.E s/N.sub.o =2(.sup.E b/N.sub.o). Because symbols are actually transmitted, it is often more convenient to measure .sup.E s/N.sub.o directly and then convert to .sup.E b/N.sub.o if desired.
When a high .sup.E s/N.sub.o or related metric, like .sup.E b/N.sub.o, is experienced, the system attempts to increase the transmit power in order to overcome the fade. However, when the radiotelephone moves out of the fading conditions, the transmit power must be decreased quickly or system capacity will suffer.
The IS-95 power control process may not be able to lower the transmit power fast enough. Fast forward link power control decreases the transmit power rapidly but relies on finding an accurate estimate of .sup.E s/N.sub.o or related metric. Additionally, there must be a way to switch between using the IS-95 power control and fast forward power control since they behave differently under different conditions. There is an unforeseen need for a process to find a reliable estimate of the bit-energy-to-noise-density or symbol-energy-to-noise-density of the channel and also be able to determine when to use the IS-95 power control and fast forward power control.